qemuBuildCommandLine: Change the condition for -nographics
    
    There's this check when building command line that whenever
    domain has no graphics card configured we put -nographics onto
    qemu command line. The check is 'if (!def->graphics)'. This
    makes coverity think that def->graphics can be NULL, which is
    true. But later in the code every access to def->graphics is
    guarded by check for def->ngraphics, so no crash occurs. But this
    is something that coverity fails to deduct.
    In order to shut coverity up lets change the condition to
    'if (!def->ngraphics)'.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>